Scientists have finally identified the mysterious source of X-ray emissions at the center of our galaxy's youngest supernova: Inside the remains of Cassiopeia A sits a baby neutron star surrounded by a thin layer of carbon. Discovered in Chandra’s “First Light” image obtained in 1999, the point-like X-ray source at the center of Cas A was presumed to be a neutron star, or pulsar, the typical remnant of an exploded star, but it surprisingly did not show any evidence for X-ray or radio pulsations.
Pulsars rank at or near the top of freaky phenomena found in our Universe. In the early 1930s, California Institute of Technology astrophysicist, Fred Zwicky, an immigrant from Bulgaria, focused his attention on a question that had long troubled astronomers: the appearance of random, unexplained points of light, new stars.
It occurred to Zwicky that if a star collapsed to the sort of density found in the core of atoms, the result would be an unimaginably compacted core: atoms would be crushed together with their electrons squeezed into the nucleus, forming neutrons and a neutron star, with a core so dense that a single spoonful would weigh 200 billion pounds. But there’s more, Zwicky concluded: with the collapse of the star there would be huge amounts of leftover energy that would result in a massive explosion, the biggest in the known universe that we called today supernovas.
Most neutron stars house incredibly large magnetic fields. If they are spinning rapidly they make fabulous clocks, cosmic radio beacons we call pulsars. Pulsars can keep time to an accuracy better that one microsecond per year. Some pulsars generate more than 1000 pulses per second, which means that an object with the mass of the Sun packed into an object 10 to 20 kilometers across is rotating over 1000 times per second, or more that half the speed of light!
Twenty times heavier than our sun and 11,000 light years away, Cassiopeia A was a dense star whose explosion was observed from Earth roughly 330 years ago. The supernova left behind a dense central core 12.5 miles wide that was first spotted in 1999 by NASA's Chandra X-ray Observatory.
But until now, astronomers hadn't come up with a model to explain the object's confusing X-ray emission spectrum. Previous attempts had come up with a radius too small to be a neutron star, or a non-uniform surface temperature, which didn't make sense.
Combining data from two prior studies, researchers have discovered that Cassiopeia's X-ray emission pattern can be explained by the presence of a very young neutron star with a low magnetic field and an unusually thin carbon atmosphere.
Cassiopeia A (Cas A, for short) the remains of a massive star that exploded in our galaxy. Evidence for a thin carbon atmosphere on a neutron star at the center of Cas A has been found. Besides resolving a ten-year-old mystery about the nature of this object, this result provides a vivid demonstration of the extreme nature of neutron stars.
By applying a model of a neutron star with a carbon atmosphere to this object, it was found that the region emitting X-rays would uniformly cover a typical neutron star. This would explain the lack of X-ray pulsations because this neutron star would be unlikely to display any changes in its intensity as it rotates. The result also provides evidence against the possibility that the collapsed star contains strange quark matter.
The properties of this carbon atmosphere are remarkable. It is only about four inches thick, has a density similar to diamond and a pressure more than ten times that found at the center of the Earth. As with the Earth’s atmosphere, the extent of an atmosphere on a neutron star is proportional to the atmospheric temperature and inversely proportional to the surface gravity. The temperature is estimated to be almost two million degrees, much hotter than the Earth’s atmosphere. However, the surface gravity on Cas A is 100 billion times stronger than on Earth, resulting in an incredibly thin atmosphere.
Casey Kazan. Adapted from materials provided by the Chandra Space Observatory.
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